In 2500 BC in Pakistan, every house had a bathroom and running water. We arrived in Europe in 1800. Mohenjo-daro, the Indus civilization. 40,000 inhabitants, grid-patterned streets, brick-covered sewers, wells in every neighborhood. Every house had a flushing toilet. Then in 1900 BC the city was suddenly abandoned. Skeletons in the streets, no war, no floods. They had the urban planning we copied 4,500 years later. And they disappeared in a day. What made them flee so quickly?